Purifying benzoic acid



Patented Apr. 27, 1943 PURIFYING BENZOIC ACID Ivan Gubelmann,Wilmington, Del., and Roland G. Benner, Carneys Point, N. J., assignorsto E. I. du Pont de Nemours & Company, Wilmington, Del., a corporationof Delaware No Drawing. Application June 27, 1940, Serial No. 342,662

11 Claims.

' This invention relates to methods of purifying benzoic acid andparticularly benzoic acid prepared by the catalytic oxidation oftoluene.

Most of the benzoic acid produced heretofore has been made by thedecarboxylation of phthalic cid. Recently, considerable progress hasbeen made in the production of benzoic acid by methods involving thecatalytic oxidation of toluene with air or oxygen-containing gas.

D. J. Loder, in his application Serial No. 235,578 filed October 18,1938, has disclosed a particularly successful method whereby alkylsubstituted aromatic compounds, particularly toluene, in a suitablesolvent, such as acetic acid, are oxidized in the liquid phase by meansof an oxygen-containing gas in the presence of a cobalt catalyst at hightemperatures. Since the two processes are quite different, differentby-products and impurities are formed which have made it necessary toevolve new methods of purifying benzoic acid made by the catalyticoxidation of toluene.

It is an object of the present invention to provide new methods forpurifying benzoic acid obtained by the catalytic oxidation of toluenewith air or other oxygen-containing gases. Another object is to provideatwo step method of purifying benzoic acid obtained by the catalyticoxidation of toluene, each of which steps are novel and may be employedseparately to obtain desirable results. A further object is to providemethods of purifying benzoic acid obtained by the catalytic oxidation oftoluene, which will produce higher yields of purer benzoic acid. Otherobjects are to advance the art. Still other objects will appearthereinafter.

The above and other objects may be accomplished in accordance with ourinvention which comprises washing an oxidation mass, obtained by thecatalytic oxidation of toluene in a solvent, with a dilute aqueoussolution of a small proportion of a basic reacting material attemperatures between the freezing point and the boiling point of theoxidation mass and subsequently distilling the crude benzoic acid in thepresence of a small proportion of sulfuric acid. While the main objectof our invention will preferably be obtained by such combination ofsteps, other objects of our invention may be accomplished by the use ofeither step alone. These methods of our invention are particularlyadapted to the purification of benzoic acid contained in oxidationmasses, obtained by the method of Loder in his application Serial No.235,578 hereinbefore referred to.

The nature and constitution of the by-products and impurities, in theoxidation mass formed by the catalytic oxidation of toluene to benzoicacid, have not been determined. They are quite complex mixtures and havenot proved amenable to ready identification. We have found that some ofthem, particularly the most undesirable, may

be washed out of the oxidation mass with an aqueous solution of a basicreacting material. If such impurities are not removed prior todistillation of the benzoic acid from the reaction mass, sodiumbenzoate, prepared from the distilled benzoic acid, is of inferiorquality. A saturated solution, containing 5 grams of U. S. P. sodiumbenzoate, should remain clear after the addition of 35 cc. of methanol.Benzoic acid, obtained by distillation from an oxidation mass which hasbeen washed with an alkaline solution in accordance with our invention,can be converted to sodium benzoate of U. S. P. quality, without furtherpurification, by dissolving it in sodium bicarbonate solution and dryingthe resulting solu tion after clarification with activated carbon. 'Ifsuch washing treatment is omitted, the sodium benzoate is of inferiorquality and a saturated solution thereof, containing 5 grams of suchsodium benzoate, becomes very turbid on addition of 35 cc. of methanol.

The step of washing the oxidation mass should usually be carried out attemperatures between the freezing point and the boiling point of theoxidation mass, that is, at temperatures above the freezing point andbelow the boiling point. This temperature range will vary with differentoxidation masses, but will usually be from about 50 C. to about C. Afterthe washing step, the oxidation mass will ordinarily be distilled,whereupon a lower boiling fraction, containing benzaldehyde, benzylalcohol, benzyl acetate and a small amount of benzoic acid, willgenerally distill off first and then the main fraction of the benzoicacid will be distilled off.

While caustic soda or sodium hydroxide is the preferred basic materialto be employed in the washing step, any other basic material, which willreact with acetic acid to form water-soluble salts therewith, will befound to be suitable and the term basic reacting material, as employedherein and in the claims, will have such meaning. The basic reactingmaterials may be of the classes of free bases, hydroxides and oxides ofthe alkali and alkaline earth metals and salts of weak acids and strongbases. Examples of such basic materials are, potassium hydroxide,ammonium hydroxide, calcium hydroxide, barium hydroxide, sodiumcarbonate, potassium carbonate, sodium acetate, calcium acetate,potassium acetate, sodium benzoate, potassium benzoate, trisodiumphosphate, sodium butyrate, potassium butyrate and the like. We havediscovered that such basic materials will react with the mostobjectionable impurities in preference to the benzoic acid and also inpreference to aliphatic acids, such as acetic acid, when such aliphaticacids-are employed as solvents, and will remove such impurities from theoxidation mass. This is an un0bviing material employed will generally befrom about 0.001 to about 0.3 of a chemical equivalent for each mole ofthe benzoic acid. Preferably, we employ from about 0.015 to about 0.03of a chemical equivalent of the basic reacting material for each mole ofthe benzoic acid. The

amount of water employed, in producing the solution of basic reactingmaterial for washing the oxidation mass, should be such that thesolution is dilute, that is, that the solution contains the basicreacting material in a concentration of less than 50%. Water is formedduring the oxidation and, in some cases, no additional water need beadded. However, better results are obtained if some water is added withthe basic reacting material, and preferably sufficient so that theconcentration of the basic reacting material in the solution fallswithin the range of from 0.5% to about The benzoic acid, in oxidationmass, may also be purified by adding thereto a small proportion ofsulfuric acid and then fractionaliy distilling the mass to recoverpurified benzoic acid. Usually, it will be found desirable to distilloff the more volatile constituents of the oxidation mass, such asbenzaldehyde, benzyl alcohol and benzyl acetate, prior to the additionof the sulfuric acid, in order to recover benzaldehyde which otherwisewould be destroyed by the sulfuric acid. The benzoic acid may bedistilled,

after the addition of the sulfuric acid, without any prolonged treatmentwith the sulfuric acid. However, it will generally be preferred toreflux the oxidation mass in the presence of the sulfuric acid for ashort period of time, up to about one hour, before carrying out thedistillation step. The refluxing may be carried out over longer periodsof time but without material advantage. Preferably, the refluxing willbe continued over a period of approximately minutes. of purifiedmaterial than is obtained if the refluxing step is omitted. Suchtreatment with sulfuric acid may be carried out, without the preliminarywashing step with a basic reacting material, to obtain purified benzoicacid which will be useful for many purposes, but which will notgenerally be suitable for making sodium benzoate of U. S. P. qualitywithout further purification. Benzoic acid, obtained by distillationfrom sulfuric acid, has a freezing point of 05 C. to 1.0 C. higher thanbenzoic acid obtained by distillation in the absence of the sulfuricacid.

The amount of sulfuric acid, to be added to the crude benzoic acidbefore distillation, may vary from about 0.1% to 20% of the weight ofthe benzoic acid. Preferably, we employ from about 0.5% to 2% ofsulfuric acid based on the benzoic acid. Sulfuric acid of anyconcentration may be employed since the water will boil off during thefirst stage of the distillation to leave concentrated sulfuric acid inthe oxidation mass during the distillation of the benzoic acid. It willgenerally be preferred to add concentrated sulfuric acid to theoxidation mass prior to the start of distillation. By concentratedsulfuric Such refluxing results in a higher yield acid, we mean sulfuricacid of 50% to strength or greater. For example, sulfuric acid,containing dissolved sulfur trioxide, known as oleum, may be employed.Particularly, we prefer to employ sulfuric acid of about 98%concentration.

While, as has been pointed out hereinbefore, benzoic acid of improvedpurity and quality may be obtained by employing either the washing stepor the sulfuric acid distillation step alone, we have found that thebest results are obtained if the steps are combined in a single processcomprising washing the oxidation mass with the solution of the basicreacting material, then distillin off the more volatile constituents ofthe oxidation mass, then adding sulfuric acid to the crude benzoic acidso produced, and then distilling off the purified benZol'c acid. By suchprocedure, benzoic acid of higher purity and higher freezing point isobtained which is particularly adapted for making sodium benzoate of U.S. P. quality.

In order to more clearly illustrate our invention, the preferred modesof carrying the same into effect and the advantageous results to beobtained thereby, the following examples are given:

Example 1 Air was passed through a solution of 1340 parts of toluene inparts of acetic acid in the presence of 1.2 parts of cobaltous acetateand 0.25 part of lead acetate at to 220 C. and at about 750 poundspressure. About 1500 parts of oxidation mass, containing about 550 partsof benzoic acid, were obtained. The oxidation mass is distilled untilthe constituents, which boil lower than benzoic acid, have been removed.5 parts of 98% sulfuric acid are then added to the distillation residuewhich is fractionated under vacuum. The main fraction has a freezingpoint of about 122 C. It is colorless in the molten state but is notsuitable for making sodium benzoate of U. S. P. quality without furtherpurification, however, it is suitable for many other purposes.

Example 2 1500 parts of an oxidation mass, obtained as described inExample 1, is washed with 200 parts of 2% caustic soda solution. Theupper layer is distilled at atmospheric pressure until the unre actedtoluene has been removed and is then fractionated under vacuum. Afterremoval of a lower boiling fraction containing benzaldehyde, benzylalcohol, benzyl acetate, and a small amount of benzoic acid, the mainfraction of benzoic acid is collected. The main fraction has a freezingpoint of 121 to 122 C. and is suitable for making sodium benzoate of U.S. P. quality without further purification.

Example 3 1500 parts of an oxidation mass, obtained as described inExample 1., was agitated with 200 parts of 2% caustic soda solution at60 to 70 C. for several minutes, after which, the lower aqueous layerwas drawn off. The lower layer contained a small amount of benzoic acidwhich was recovered by extraction with toluene. The upper layer wasdistilled to remove the toluene and then fractionated under vacuum torecover benzaldehyde, benzyl alcohol and benzyl acetate. After removalof these materials, 5 parts of 98% sulfuric acid was added to the stillresidue which was crude benzoic acid. The mass was refluxed for aboutminutes and then the benzoic acid was distilled. The main fraction had afreezing point of above 122 C. and was suitable for making sodiumbenzoate of U. S. P. quality without further purification.

It will be understood that the above examples are given for illustrativepurposes only and that many variations and modifications may be madetherein, as pointed out hereinbefore in the specification. Still othermodifications and variations, which may be made in our process withoutdeparting from the spirit of our invention, will be apparent to thoseslnlled in the art. Accordingly, our invention is not to be limited bythe specific embodiments given in the examples, but we intend to coverour invention broadly as in the appended claims.

We claim:

1. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise adding a small proportion of sulfuric acid to the crudebenzoic acid and then fractionally distilling to recover purifiedbenzoic acid.

2. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise distilling off components of the oxidation mass which aremore volatile than benzoic acid, then adding a small proportion ofsulfuric acid to the crude benzoic acid and then fractionally distillingto recover purified benzoic acid.

3. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkaline material at temperatures betweenthe freezing point and the boiling point of the oxi dation mass, thenadding a small proportion of sulfuric acid to the crude benzoic acid andthen fractionally distilling to recover purified benzoic acid.

4. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkaline material at temperatures betweenthe freezing point and the boiling point of the oxidation mass, thendistilling ofi components of the oxidation mass which are more volatilethan benzoic acid, then adding a small proportion of sulfuric acid tothe crude benzoic acid and then fractionally distilling to recoverpurified benzoic acid.

5. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkali metal base at temperatures betweenthe freezing point and the boiling point of the oxidation mass, thenadding a small proportion of sulfuric acid to the crude benzoic acid andthen fractionally distilling to recover purified benzoic acid.

6. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkali metal base at temperatures betweenthe freezing point and the boiling point of the oxidation mass, thendistilling ofi components of the oxidation mass which are more volatilethan benzoic acid, then adding a small proportion of sulfuric acid tothe crude benzoic acid then fractionally distilling to recover purifiedbenzoic acid.

7. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof from about 0.015 to about 0.03 chemical equivalent of an alkali metalbase at temperatures between the freezing point and the boiling point ofthe oxidation mass, then distilling oif components of the oxidation masswhich are more volatile than benzoic acid, then adding a smallproportion of sulfuric acid to the crude benzoic acid and thenfractionally distilling to recover purified benzoic acid.

8. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkaline material at temperatures betweenthe freezing point and the boiling point of the oxidation mass, thenrefluxing the crude benzoic acid for a short period of time in thepressure of a small proportion of concentrated sulfuric acid and thenfra-ctionally distilling to recover purified benzoic acid.

9. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkaline material at temperatures betweenthe freezing point and the boiling point of the oxidation mass, thendistilling off components of the oxidation mass which are more volatilethan benzoic acid, then refluxing the crude benzoic acid for a shortperiod of time in the presence of a small proportion of concentratedsulfuric acid and then fractionally distilling to recover purifiedbenzoic acid.

10. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of an alkali metal base at temperatures betweenthe freezing point and the boiling point of the oxidation mass, thendistilling off components of the oxidation mass which are more volatilethan benzoic acid, then refluxing the crude benzoic acid for a shortperiod of time in the presence of a small proportion of concentratedsulfuric acid and then fractionally distilling to recover purifiedbenzoic acid.

11. In the process of purifying benzoic acid in an oxidation mass,obtained by the catalytic oxidation of toluene in a solvent, the stepswhich comprise washing the oxidation mass with a dilute aqueous solutionof a small proportion of sodium hydroxide at temperatures between thefreezing point and the boiling point of the oxidation mass, thendistilling off components of the oxidation mass which are more volatilethan benzoic acid, then refluxing the crude benzoic acid for a shortperiod of time in the presence of a small proportion of concentratedsulfuric acid, and then fractionally distilling to recover purifiedbenzoic acid.

IVAN GUBELMANN. ROLAND G. BENNER.

